1. Field of the invention
The present invention relates generally to microwave slot array antennas. More particularly the invention relates to a waveguide slot array system employing a termination at the center of the array which is inoperative when the array is excited in a sum mode and which is operative when the array is excited in a difference mode, thereby achieving a different array excitation than is customary when only the phase is changed for each mode.
2. Description of Related Art
Wavequide-fed slot radiators and slot array antennas are widely used in microwave communications and radar applications. In these applications, it is frequently important to design the antenna to have minimal side lobe patterns in both the horizontal and vertical planes, as this will help reduce the communications or radar system's susceptibility to jamming and interference. Many of today's microwave systems utilize both sum (in-phase) and difference (out-of-phase) excitation of the antenna aperture. In a monopulse tracking radar, for example, the sum and difference excitations provide different signals which can be compared and analyzed with respect to amplitude and phase in order to determine the spatial position of a target. In such radar systems, it is desirable that the antenna have minimal side lobes in both planes for both modes of excitation. This has been difficult to achieve in practice, since the excitation which lowers the sum mode side lobe pattern increases the difference mode side lobe pattern in conventional array antennas.
Traditionally, the approach to slot array antenna design has been to design the antenna for optimum side lobe pattern for sum mode excitation and then be willing to accept whatever difference mode side lobe pattern results. Traditionally, there has been no effective way of providing independent control over the side lobe patterns for the sum and difference modes separately.
One approach to this design problem which has been considered is that described in my U.S. Pat. No. 2,981,948 which issued on Apr. 25, 1961, entitled "Simultaneous Lobing Array Antenna System," assigned to the assignee of the present invention. That antenna system employs a directional coupler or hybrid junction to achieve a unidirectional subdivision of the antenna array. Although useful in providing simultaneous lobing, that antenna is not practical in a waveguide-fed slot array for use in certain applications such as missile seeker applications and flat-plate seeker antennas. Strip line-fed slots and dipoles have also been used to achieve independent aperture distributions for sum and difference excitation, but these have the disadvantages of complex fabrication and higher loss, to name but two.